Timing doesnt advance

[airemdwn]

1991 Blazer, driving home yesterday, power starts cutting out at any and all rpms. i was thinking fuel but nursing it home, it felt ignition. It did not throw a code i checked it tonight and the timing does not move its barely running and the timing is stuck at 0*. I tries unplugging the EST wire that just threw a code when i plugged it back in.
The distributor was rebuiltless than 1000 miles ago, New Delco pickup coil and delco Module. Is it the module that went bad and isnt advancing or could it be something else?

 

[ColoradoChevyK5]

Seems even with no advance the initial timing should be around 12 degrees or so. Did the distributor turn a bit? See if the hold-down clamp is tight.

 

[eagle mark]

Did the bypass wire come disconnected? That will cause 0 degrees timing and how you set timing to 0. It will cause an DTC Error 42 but can be cleared after timing is set.

EST module is what handles timing. When it was installed did you use the heat disapating compound underneath? If not they will get hot and fry... check screws holding the module down as they are the grounds.

 

[airemdwn]

Changed the electronic spark control module, (knock sensor) not sure i guess i had it for so long i just wanted to change it. Didnt work so back in the spare parts bin it goes.
Next was the Ignition module, used new Napa brand instead of used GM module, that wasnt it either.

Swapped ECM. Fixed
So now i have to carry another ECM?
Im going to need more storage.........

 

[eagle mark]

Very rare to have an ECM go bad after so many years. They usually fail when very new.

You sure there was not a error code set leaving you in limp home mode? That would cause the issue and changing the ECM, or just unplugging the old one would clear code.

I've been driving these TBI ECM trucks since 1987 and never had an ECM fail.

 

[Fordum]

I don't know how hard it is to change the ECM, never worked on one in a GM, but if its not too hard, I would try swapping it back briefly.
If it was in limp home mode, removing it would have cleared that, and if it was just a bad connection in the plug, unplugging it and replugging it might have fixed that.

Of course, if you swap it back and it works, then you have the problem of which one you run.........

Be sure to look at the plug on the ECM as well as the wiring harness for a loose pin or connector too.
Ignition modules, pickups, even coils go out often, ECMs seem to go on forever.

 

[airemdwn]

What is "limp home mode"?

I did not send a code during any drive which made me suscipious, it was running poorly and i made a couple round the block trips to feel what was happening. When i suspected timing and saw it at 0* i unpluggeed the EST wire which did nothing but then throw the 42 code.

I was surprised the ecm was bad also. i converted my 74 and a friends 75 and have been running GM TBI since 1998. The motor is a 383, The computer is a 1227747 out of a ??? i dont even remeber but its not out of the 91, it has a custom chip so maybe it contributed to its death. But its running fine again.

 

[Fordum]

Limp home mode, may not exist for your truck. It applies to electronic controlled engines and transmissions when something goes out of range and the computer cannot correct it or figure out what to do.

It just sets things up so that the truck will keep going, but be least likely to do any damage.

For instance, in computer controlled transmissions, it usually locks it in second gear, no torque converter lockup.
You still have reverse, but it will only drive forward in second.
This lets you know you have a problem, but keeps you from driving far or fast.

In this case, the computer may have lost the feedback that told it how much advance the engine was getting, or maybe the knock sensor showed bad.
If the computer could not know if the engine was knocking, it would lock out the advance to keep from damaging the engine.

Also, if it has a custom chip, then there is no telling what might have told it to stop advancing.
The person who burned the chip could have put in no advance for any number of reasons.

 

[eagle mark]

Limp Home Mode is a fail safe so the engine will run if catastrophic failure. That is what the little chip is for. But with no check engine light or codes I don't think it is possible?

A 1227747 is a 1227747. A custom chip can not cause it to fail. It was running correctly right? Then this happened? It sounds like you actually had an ECM failure! Still very rare! I have seen it happen after welding on a vehicle while grounds are still connected, but even that is rare. Or when they get wet with power applied.

Can you think of anything that you did or happened just before this? I'm curious.

 

[airemdwn]

Im having AC Issues, a short cycling compressor.

I was also pretty shocked at the heat on the ecm when i took it out. This is the first factory intsall im dealing with for modifying and it was really hot and i wasnt runnning it that long. i must assume its normal but i put some reflective heat shield between the heater box and the puter now just for my piece of mind. I doubt heat was a factor in the failure.
I am fairly certain that this was a junkyard ecm I had a few in the garage when i was in the build and who knows how long it was siiting in the yard not running kinda in the weather, and my build was almost 2 years in an unheated garage.

 

[eagle mark]

I saw your AC thread... OK there is a signal wire so ECM knows it's on... I wonder? I doubt it because I had low pressure in mine once and it cycles and no issue... it does have some IAC commands with AC, but no timing.

So did the ECM ever work? I've got bad ones from JY, but never had a good one fail. They also can get soaking wet and still work if dried before power is turned on. Pretty robust peice of anceint electronics. Like I said they did fail when vehicle was new, it was just a matter of if you got one with a bad solder or? that failed. But after they run and shake going down the road they usually last forever. GM service test is to tap on ECM while idling to see if there's a glitch.

FYI the GM EST is accurate, aftermarket EST can be 11 degees off timing at who knows what RPM?